1. Field of the Invention
The present invention relates to a navigation apparatus that searches for a route to a destination and guides a vehicle to the destination according to the guidance route searched for. More particularly, the invention is directed to an access control of a hard disk drive incorporated in a navigation apparatus when a vehicle is traveling areas located at high altitude.
2. Description of the Related Art
Navigation systems are designed to detect the present position of a vehicle to thereby read map data covering the present vehicle position and its surroundings from a storage medium, and to draw a map on a screen of a display with a vehicle mark superimposed on the map. In the systems, when the vehicle is moved and the present position thereof is changed, the map is scrolled accordingly, or the vehicle mark is moved on the map accordingly. This provides a user with easy to understand map information about the periphery of the vehicle position.
The navigation systems have the function of searching for a route to a destination set by a user and guiding the vehicle along the route. The route guiding function is to search for or retrieve a cost-effective route connecting from the present vehicle position to the destination by the Dijkstra method or the like using map data, to store the route searched for in a memory as guidance route data, and to display the guidance route on a display device during traveling of the vehicle.
In recent years, some navigation systems incorporate an audio function as well as a navigation function. Such a system can be used to reproduce both a CD-ROM for music and a CD-ROM which stores therein information on maps or the like needed for the navigation. Thus, there is a disadvantage that when reproducing one storage medium, the system cannot reproduce the other storage medium.
To eliminate the above mentioned disadvantage, Japanese Unexamined Patent Publication No. 9-62183 discloses navigation technology in which, even when a CD-ROM storing map data or the like is not loaded onto the vehicle, the map data is stored in another storage means, and a navigation function is available using the map data or the like stored in the storage means. Japanese Unexamined Patent Publication No. 2001-221645 discloses a navigation system that previously stores music data in a music data storage means to thereby perform a navigation function while reproducing music.
The emergence of a hard disk drive capable of storing large quantities of data encourage or stimulate the commercialization of navigation systems with the hard disk drive used as a storage medium. This drive has enough capacity to store not only data on maps but also different data, e.g., data on music, programs, and the like. The use of this hard disk drive as a single storage device permits concurrent execution of the navigation function and the audio function, thereby eliminating the disadvantage in utilizing a plurality of CD-ROMs as mentioned above.
However, the navigation systems using the hard disk drive have the following disadvantages. In order for the hard disk to store data in high density, a clearance between a magnetic head and the disk should be extremely small. In this system, a pressure caused by the viscosity of air which is generated by rotation of the disk causes the magnetic head to float above the disk. In a low atmospheric pressure, the clearance between the magnetic head and the disk cannot be maintained, which might result in destruction of the disk. In general, the hard disk guarantees an operating environment of an altitude up to about 3,000 m. Thus, when the vehicle with the navigation system mounted thereon is traveling roads at an altitude of about 3,000 m or above, e.g., 4,000 m or 5,000 m category (roads in the U.S.A., China, or the like), the hard disk might be damaged, leading to a failure in the navigation system.